I've tried this. The best solution I came up with was one of those reasonably good quality tarps from Home Depot or whatever, 12' x 10' I think, and 4 bungee cords.
I throw the tarp over the cab, hook the two front corners UNDER the side mirrors, and stretch the bungee cords around the front of the truck and hook them into the edge of the grille. Then I pull the tarp to the back, hook the left 2nd to last hole with the bungee, run it under my trailer hitch over to the right side, and then up the to right side LAST hole. Then I do the opposite on the right side to the left. Therefore, on the back the tarp is held with FOUR of the holes. It looks kind of like two V's on the back.
This keeps a nice tight tension across the top of the cab to the back tailgate. When it rains the water runs down and off the side.
Now, if you make a mistake and park on a downhill, the water WILL pool on the tarp and it WILL leak through. Be sure you don't do that. Or you don't mind pulling up the carpet to dry it out for a week or two ...
Anyway, the tarp is easy to roll up and shove under the drivers seat. I cannot say the same about a true car cover.